In packet-oriented mobile communication systems such as 3GPP Long Term Evolution (LTE) and High Speed Packet Access (HSPA), a terminal must repeatedly monitor the transmitted downlink signals to see whether there are any transmissions directed to the terminal. A downlink transmission comprises a control message and a data packet, where the control message indicates the presence of the data packet as well as variable formatting details that are needed for receiving the data packet. The control message can also have a variable formatting, and the terminal typically has to search through a list of possible formatting alternatives of the control message, and for each control message candidate, try to decode it. If a control message is found, the terminal tries to receive the data packet using the formatting details from the control message. If no control message is found, the terminal can disable its receiving circuitry until the next opportunity to receive a downlink transmission, thereby saving processing power.
In the case of LTE, the control message is called Downlink Control Information (DCI) and is transmitted over the Physical Downlink Control Channel (PDCCH), while the data packet is transmitted over the Physical Downlink Shared Channel (PDSCH). Transmission opportunities occur with a period of 1 ms, called subframes.
Searching for the control messages takes time. This means that the terminal does not know immediately whether it will receive a data packet or not; the terminal does not know it until it has found a DCI specific for it. Therefore, the terminal has to continue receiving and buffering the radio signal after having received the control transmission, in case it finds a control message within the control transmission that indicates that there is a downlink data transmission intended for it. In many scenarios with low to moderate load, a majority of the transmission opportunities are unused, in the sense that there are no control messages and no data transmissions to be found. Still, all active terminals have to spend considerable resources in receiving and buffering the radio signals, and searching for control messages.
In LTE the terminal does not know the exact location of the PDCCH within a downlink control region. A PDCCH may consist of 1, 2, 4, or 8 control channel elements (CCEs) that each contains 36 resource elements. Each PDCCH supports multiple formats and the format used is a priori unknown to the terminal. A scheduler of a base station dynamically decides both the PDCCH format as well as which CCEs that shall be used when transmitting a DCI message to a terminal. To impose as few restrictions as possible on the scheduler while still reducing the maximum number of blind decoding attempts required by the terminal to find the PDCCH, LTE defines so-called search spaces which describe the set of CCEs that the terminal is supposed to monitor. In order for a certain terminal to determine that there is no PDCCH inside the downlink control region intended for it, the terminal needs to try all combinations in the search space and check that no one can be decoded.
It is important to minimize the power consumption of the terminal, offering the user a terminal with as long battery life time as possible. FIG. 1 illustrates an example of energy consumption of a terminal in idle mode when nothing is transmitted. For many terminals the energy consumption in the receiving parts will be dominating simply because the terminal does not transmit very often. It can be seen in FIG. 1 that the receiver (RX) radio front-end is dominating and that it is particularly expensive, in terms of power consumption, for the terminal to have the RX front-end activated. Typically the terminal needs to activate the RX front-tend when receiving data, when listening for paging transmissions from the communication network, and when performing radio measurements. During the time the terminal searches through the search space for downlink control information the RX front-end needs to be active since the terminal must buffer the received signal in case it contains any data transmission for the terminal.